Optical interleaver

ABSTRACT

In accordance with another exemplary embodiment of the present invention, an optical device includes an interleaver/deinterleaver, which includes a passive thermal compensator, wherein an optical signal which traverses the optical device undergoes substantially no temperature induced frequency drift over a desired temperature range.  
     In accordance with another exemplary embodiment of the present invention, an optical device includes a first element which decomposes said optical signal into a first beam and a second beam with the first beam being in a first polarization state and the second beam being in a second polarization state. The first and second polarization states being orthogonal to one another and including each of said multiple channels. The optical device further includes a second element which transforms the first beam into a first elliptically polarized state having odd channels in a third polarization state and even channels in a fourth polarization state and the second element transforming the second beam into a second elliptically polarized state having even channels in the third polarization state and odd channels in the fourth polarization state; and a third element which combines the odd channels into a first output port and the even channels into a second output port.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a continuation-in-part applicationunder 35 USC § 120 of U.S. Patent Application Serial Number (Atty.Docket Number Krol 6A/Crng.038) filed Mar. 20, 2002 and entitled“Tunable Optical Filter,” which is a continuation of U.S. patentapplication Ser. No. 09/356,217 filed Jul. 16, 1999. The presentapplication also claims priority under 35 USC § 119(e) from U.S.Provisional Application Ser. Nos. 60/279,695 and 60/279,696, entitled“Optical Interleaver”, and “Optical Interleaver With Passive ThermalEffect Compensation,” respectively, and filed on Mar. 30, 2001. Thedisclosures of all of the above referenced patent applications andprovisional applications are specifically incorporated herein byreference as though reproduced in their entirety herein.

FIELD OF THE INVENTION

[0002] The present invention relates generally to opticalcommunications, and particularly to an opticalinterleaver/deinterleaver.

BACKGROUND OF THE INVENTION

[0003] The increasing demand for high-speed voice and datacommunications has led to an increased reliance on opticalcommunications, especially optical fiber communications. The use ofoptical signals as a vehicle to carry channeled information at highspeed is preferred in many instances to carrying channeled informationat other electromagnetic wavelengths/frequencies in media such asmicrowave transmission lines, coaxial cable lines, and twisted copperpair transmission lines.

[0004] Advantages of optical media include higher channel capacities(bandwidth), greater immunity to electromagnetic interference, and lowerpropagation loss. In fact, it is common for high-speed optical systemsto have signal rates in the range of approximately several megabits persecond (Mbit/s) to approximately several tens of gigabits per second(Gbit/s), and greater. However, as the communication capacity is furtherincreased to transmit greater amounts of information at greater ratesover fiber, maintaining signal integrity can be exceedingly challenging.

[0005] One way to more efficiently use available resources in the questfor high-speed information transmission is known as multiplexing, inwhich a plurality of channels are transmitted along an optical waveguide(e.g. an optical fiber). One particular type of multiplexing iswavelength division multiplexing (WDM). In WDM, each high-speedinformation channel has a center wavelength and prescribed channelbandwidth. At the receiver end, the plurality of optical channels isthen separated and may be further processed by electronics. (Byconvention, when the number of channels transmitted by such amultiplexing technique exceeds approximately four, the technique isreferred to as dense WDM or DWDM).

[0006] While transmission of information via an optical medium hasoffered significant improvements in information transmission, increaseddemand for capacity may still adversely impact signal quality duringtransmission. For example, the number of channels that can be carried ina single optical fiber is limited by cross-talk, narrow operationbandwidth of optical amplifiers, and optical fiber non-linearities.

[0007] Current wavelength channel center wavelengths, channel bandwidthsand spacing between interleaved channels corresponding wavelengthspreferably conform to an International Telecommunication Union (ITU)channel grid. For example, one ITU channel grid has a channel spacingrequirement of 100 GHz. In this case, the channel spacing is referencedin terms of a frequency spacing, which corresponds in this example to achannel center wavelength spacing of 0.8 nm. With 100 GHz channelspacing, channel “n” would have a center frequency 100 GHz less thanchannel “n+1” (or channel “n” would have a center wavelength 0.8 nmgreater than the center wavelength of channel “n+1”).

[0008] As can be appreciated, the more information that is sent over aparticular medium, the greater the number of channels that are needed.It follows, that due to bandwidth considerations, the larger the numberof channels, and the closer the separation between channels become.Among other difficulties, the decrease in channel spacing makesseparating the plurality of optical channels more challenging. Forexample, in order to preserve the integrity of the signal at thereceiver end of the communication link, cross-talk in the form ofreceived channel overlap must be minimized. As can be appreciated,meeting these performance requirements of ever-increasing demand is atechnical and practical challenge.

[0009] The technical and practical challenges described above arefurther exacerbated by environmental factors. These environmentalfactors can adversely impact the performance of the devices. Onedeleterious environmental factor is the ambient temperature. Forexample, in the change in the ambient temperature can create temperatureinduced wavelength drift of the WDM. This wavelength drift can causewavelength channel overlap. In the closely spaced channels of ITU gridsdiscussed above, optical system performance may be adversely impacted.

[0010] Accordingly, there is often a need to compensate for temperaturefluctuations in WDM systems. While it may be possible to control theambient temperature surrounding the WDM device, this generally requiresrather elaborate climate control devices, which can be relativelycomplex and expensive. Moreover, these devices do not ensure theparticular elements of a WDM are immune to temperature fluctuations. Assuch, in addition to adding complexity and expense, known activetemperature control schemes may be unreliable.

[0011] Accordingly, what is needed is an opticalinterleaver/deinterleaver that caters to immediate and future needs forhigh speed optical networks without the disadvantages associated withcurrent components and approaches.

SUMMARY OF THE INVENTION

[0012] In accordance with another exemplary embodiment of the presentinvention, an optical device includes an interleaver/deinterleaver,which includes a passive thermal compensator, wherein an optical signal,which traverses the optical device, undergoes substantially notemperature induced frequency drift over a desired temperature range.

[0013] In accordance with another exemplary embodiment of the presentinvention, an optical interleaver/deinterleaver includes a first elementwhich decomposes said optical signal into a first beam and a second beamwith the first beam being in a first polarization state and the secondbeam being in a second polarization state. The first and secondpolarization states being orthogonal to one another and including eachof said multiple channels. The optical interleaver/deinterleaver furtherincludes a second element which transforms the first beam into a firstelliptically polarized state having odd channels in a third polarizationstate and even channels in a fourth polarization state and the secondelement transforming the second beam into a second ellipticallypolarized state having even channels in the third polarization state andodd channels in the fourth polarization state; and a third element whichcombines the odd channels into a first output port and the even channelsinto a second output port.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The invention is based understood from the following detaileddescription when read with the accompanying drawing figures. It isemphasized that the various features in the drawing figures may notnecessarily be drawn to scale. In fact, the dimensions of the variousfeatures may be arbitrarily increased or decreased for clarity ofdiscussion. Moreoever, wherever possible, like reference numerals referto like elements.

[0015]FIG. 1 is a schematic view of an interleaver according to anexemplary embodiment of the present application.

[0016]FIG. 2 is a schematic view of an interleaver according to anotherexemplary embodiment of the present invention.

[0017]FIG. 3 is a schematic representation of an interleaver accordingto another exemplary embodiment of the present invention.

[0018]FIG. 4 is a block diagram of an interleaver according to anexemplary embodiment of the present invention.

[0019]FIG. 5 is a block diagram of an interleaver according to anotherexemplary embodiment of the present invention.

DEFINED TERMS

[0020] As used herein “interleaving” refers to combining two or morestreams of optical signals, wherein each stream contains a plurality ofoptical channels; and “de-interleaving” refers to separating an opticalsignal, which contains a plurality of optical channels, into two or morestreams of optical signals, each of which contains a subset of theplurality of optical channels. Generally interleaving decreases thechannel spacing between adjacent channels, and de-interleaving increasesthe channel spacing between adjacent channels.

DETAILED DESCRIPTION

[0021] The invention will now be described more fully with reference tothe accompanying drawing figures, in which exemplary embodiments areshown. In the following detailed description, for purposes ofexplanation and not limitation, exemplary embodiments disclosingspecific details are set forth in order to provide a thoroughunderstanding of the present invention. However, it will be apparent toone having ordinary skill in the art having had the benefit of thepresent disclosure, that the present invention may be practiced in otherembodiments that depart from the specific details disclosed herein.Moreover, descriptions of well-known devices, methods and materials maybe omitted so as to not obscure the description of the presentinvention.

[0022] Briefly, the invention is drawn to a method and apparatus forinterleaving/deinterleaving optical signals based on polarizationinteferometry. Illustratively, the input optical signal from an inputport has a known channel frequency period or channel spacing and theoutput optical signals from two output ports have a channel frequencyperiod or channel spacing that is an integer even multiple (e.g. two) ofthat of the input signal. Moreover, the illustrative embodiments hereindescribe the de-interleaving function. Of course, from the reciprocityprinciple of optics, the methods and apparati of the exemplaryembodiments present invention described herein may be used to achieve aninterleaving function. In such a case, the interleaved optical signalwould have channel spacing that is an even fraction (e.g. one-half) ofthe channel spacing of the two input optical signals.

[0023] Turning initially to FIG. 1, an interleaver/deinterleaver 100(hereinafter referred to as interleaver 100) according to an exemplaryembodiment of the present invention is shown. An input signal 101 isincident on a first element 102. The input signal 101 is illustrativelya multiplexed optical signal having channels 1,2, . . . . n, withrespective channel center wavelengths of λ₁, λ₂, λ_(n). In the exemplaryembodiment of FIG. 1, the input signal 101 is polarized light. The firstelement 102 separates the polarized light into orthogonal polarizationcomponents, which emerge as separate optical beams 103 and 104. Opticalbeam 103 is linearly polarized light having polarization vectors thatare in a first plane. Optical beam 104 is linearly polarized light aswell, having of polarization vectors that are in a second plane, whereinthe first and second planes are mutually orthogonal. Moreover, the firstelement 102 merely separates the polarization components of thepolarized input signal 101, and therefore each optical beam 103 and 104includes all of the channels 1,2,. . . n.

[0024] Next, a second element 105 transforms optical beams 103 and 104into polarized beams 106 and 107. Illustratively, polarized beam 106,which is the polarization transformation of optical beam 104, hasorthogonal polarization components, with a first polarization stateincluding the odd channels of the input signal (i.e. channels 1, 3,. . .n) and the second polarization state including the even channels (i.e.channels 2, 4, . . . n). Likewise polarized beam 107, which is thepolarization transformation of beam 104, the first polarization stateincludes even channels of the input signal 101, and the secondpolarization state includes the odd channels of the input signal 101. Itis important to note that second element 105 also increases the channelspacing (e.g. the frequency spacing) of the channels, illustratively bya factor of two. Next, a third element 108 combines the odd channelpolarization vectors of beam 103 with the odd channel polarizationvectors of beam 104, and directs the optical beam to odd output 109.Moreover, third element 108 combines the even channel polarizationvectors of beams 103 and 104 and directs them to an output port 110.

[0025] As can be appreciated from a review of FIG. 1, the input signal101, having a first channel spacing, undergoes a transformation by theinterleaver 100 into two output ports (109, 110), each of which havingchannels with a second channel spacing that is twice that of the firstchannel spacing. Moreover, outputs 109 and 110 could become the inputport of a concatenated interleaver/deinterleaver (not shown), which issimilar to interleaver 100, and the channel spacing could be increasedby another factor of two. Of course, further interleavers/deinterleaverscould be concatenated, with each increasing the channel spacing by afactor of 2. It is further noted that the outputs 109 and 110 could alsobe selectively coupled to other devices such as demultiplexers andoptical add/drops.

[0026] Turning to FIG. 2, an interleaver/deinterleaver 200 (hereinafterreferred to as interleaver 200) according to another exemplaryembodiment of the present invention is described. An input opticalsignal 201 is illustratively from an optical fiber (not shown). Theinput signal 201 is illustratively elliptically polarized, which is afair generalization since the polarization state of an optical beam canchange as it traverses an optical fiber due factors such asstress-induced birefringence. Input signal 201 is comprised ofmultiplexed optical wavelength channels, with each channel having acharacteristic center wavelength (and thereby frequency). Each channelmay also have a particular spread or sub-band, and the individualchannels are separated by a particular channel spacing. By convention,this channel spacing refers to a channel center wavelength-to-channelcenter wavelength for adjacent channels.

[0027] For purposes of illustration, channel spacing (which may be inunits of frequency period or spacing) between the individual channels inthe multiplexed input signal is within a chosen ITU grid. The channelspacing (in this case frequency spacing) may be 400 GHz, 200 GHz,100GHz, 50 GHz or 25 GHz corresponding to channel center wavelength spacingof 3.2 nm, 1.6 nm or 0.8 nm , 0.4 nm and 0.2 nm, respectively.

[0028] Of course, the above referenced specifications of channel spacingare merely illustrative and are not intended to be in any way limitingof the application of the invention. To this end, as would be readilyunderstood by one of ordinary skill in the art, the number of channelsis basically limited to the total bandwidth of an amplifier used in theoptical system. For example, this bandwidth limitation would be thebandwidth limitation of an amplifier such as an erbium doped fiberamplifier (EDFA). Illustrative wavelength channels are the C-band, whichhas typical channel wavelengths between 1528 nm and 1560 nm, and theL-band, which has typical channel wavelengths from 1565 nm to 1620 nm.Of course, the useful bandwidth for each channel is generally less thanthe spacing between each of the channels. Finally, it is noted that theinterleaver 200 of the exemplary embodiment of the present disclosuremay be incorporated into a synchronous optical network (SONET network),which complies with the OC-48 standard (2.5 Gb), the OC-192 standard (10Gb) or the OC-768 standard (40 Gb). Of course, applications of thepresent invention to these standards are merely illustrative, and otherstandard optical networks within the purview of one having ordinaryskill in the art can benefit from the use of the present invention.

[0029] The input optical signal 201 is incident perpendicularly to theend face of the first polarization splitter 202, which is illustrativelya birefringent material such as rutile, calcite and yttrium vanadate(YVO₄). For reasons discussed more fully below, the birefringence, themagnitude of the difference between the ordinary and extraordinaryindices of refraction, is usefully as great as possible. In theillustrative case where yttrium vanadate is used, the birefringence ison the order of 0.211 at a center wavelength of 1550 nm.

[0030] The first polarization splitter 202 fosters polarizationdiversity and, ultimately enables the interleaver 200 of the presentinvention to function substantially independently of the inputpolarization of the input optical signal 201. To this end, the inputoptical signal 201 may have a variety of polarization states; generally,it is elliptically polarized light. The first polarization splitter 202separates the input optical signal 201 into its component polarizationvectors (polarization states), which are then selectively transformedvia the de-interleaving process as described herein. In this exemplaryembodiment beam 203 is polarized parallel to the plane of the paper andbeam 204 polarized perpendicularly to the plane of the paper.

[0031] In the exemplary embodiment shown in FIG. 2, the firstpolarization splitter 202 is a birefringent material having a principalplane oriented parallel to the top and bottom surfaces of the firstpolarization splitter 202. The input optical signal 201, beingillustratively elliptically polarized, has two orthogonal electric fieldvectors, and components thereof oriented parallel to the extraordinaryaxis of the birefringent crystal will travel through the crystal at afaster (or slower, depending on the index of refraction, n_(e)) phasevelocity than the electric field component oriented along the ordinaryaxis.

[0032] Thus, upon emerging from the first polarization splitter 202, theinput optical signal 201 is split into two beams 203 and 204, withorthogonal polarization states. These polarization states are oftenreferred to as s and p polarization states having a relative phase thatis determined by the thickness and material properties (birefringence)of the first polarization splitter 202. The walk-off or spatialseparation of the beams 203, 204 is dependent upon the length (L) of thefirst polarization splitter 202 and its birefringence, and should belarge enough to prevent the beams 203,204 from overlapping.

[0033] As discussed above, the input optical signal 201 is multiplexedhaving channels 1-n, and the channels have respective channel centerwavelengths (λ₁, λ₂, λ_(n)). The first polarization splitter 202 merelysplits the electric field vector of beam 201 into its orthogonalcomponents, the polarization states of beams 203 and 204. It followstherefore that each beam (203 and 204) contains all of the channels.Beams 203,204, which each include odd and even channels, are thenincident on polarization transforming (PT) element 205. PT element 205is illustratively a birefringent crystal, such as calcite, rutile oryttrium vanadate. The physical properties that are desirable in element205 are its optical anisotropies for effecting the polarizationtransformation of beams 203,204.

[0034] As will become clearer as the present description proceeds, thepolarization transformation of beams 203,204 by birefringent element 205is useful in de-interleaving the input optical signal 201 into outputoptical signals having a channel spacing that is twice that of thechannel spacing of the input optical signal. Accordingly, it is theanisotropic optical properties of element 205 which are useful toachieving this desrired end. As such, while PT element 205 isillustratively a birefringent crystal, PT element 205 may be a knownphase retarder. Moreover, it is noted that various optically anisotropicmaterials could be used as element 205. These include, but are notlimited to, known electro-optic elements and birefringent optical fiber.

[0035] In the interest of simplicity of discussion, only thepolarization transformation of beam 203 will be discussed in detail. Thephysics of the polarization transformation of beam 204 is the same, ofcourse. Beam 203 is linearly polarized light having components that arein the plane of the paper as shown in FIG. 2. As described above, theinput optical signal 201 is multiplexed having channels 1-n, and thechannels have respective channel center wavelengths (λ₁, λ₂, λ_(n)); andaccordingly, the beam 203 has polarization components representing eachof these channels as well. In the illustrative embodiment, the PTelement 205 is a birefringent crystal having its principal section andc-axis oriented diagonally (at a 45° angle) relative to the end faces ofthe crystal. Stated differently, the c-axis of element 205 is orientedat a 45° angle relative to the ordinary (o) and the extraordinary (e)axes. As such the c-axis is at a 45° angle to the plane of polarizationof beam 203. Accordingly, linearly polarized light such as beam 203 willemerge as elliptically polarized light from the crystal. The benefitderived from the birefringence is the phase separation of thepolarization vectors corresponding to the individual channels 1-n withinthe beam 203.

[0036] As discussed below, the phase separation between successivechannels is dependent on the thickness (or length, L) of thebirefringent material and the index of refraction differential betweenthe ordinary and extraordinary axes. In the illustrative embodimentshown generally in FIG. 2, this phase differential between thesuccessive channels is π. Accordingly, channel 2 having a center channelwavelength λ₂ has a polarization vector upon emerging from the PTelement 205 that has a relative phase of π radians with respect to thepolarization vector of channel 1, having a center wavelength λ₁ Thereby,the first polarization splitter 202 effectively separates the channelsin beam 203 so that the odd channels have co-linear polarization vectors(i.e. linearly polarized parallel or anti-parallel to one another) andthe even channels have co-linear polarization vectors.

[0037] In the illustrative embodiment shown in FIG. 2, the odd channels206 of beam 207 have polarization vectors that are perpendicular to theplane of the paper, while the even channels 208 of beam 206 havepolarization vectors that are in the plane of the paper. By similaranalysis, beam 209, which is the polarization transformation of beam204, has odd channels 210 with polarization states in the plane of thepaper and even channels 211 with polarization states perpendicular tothe plane of the paper.

[0038] The beams 207,209 of elliptically polarized light havingorthogonal polarization vectors of even and odd channels therein arethen impingent upon the second polarization splitter 212.Illustratively, second polarization splitter 212 is a birefringentmaterial having the same thickness, optic axis orientation, and indexdifference between the ordinary and extraordinary axes as firstpolarization splitter 202. As such, the odd channels 206 from beam 207emerge undisplaced from the second polarization splitter 212 as beam 213having the polarization state shown. The odd channels 210 of beam 209,being linearly polarized vectors in the plane of the paper are displaceddue to the birefringence of the second polarization splitter 212. Shownas beam 214 with polarization states in the plane of the paper, theseodd channels along with odd channels of beam 213 are then reflected byreflective element 215 and form the odd channel output signal 216.

[0039] Similarly, the beam 207 has polarization components 208 that areoriented in the plane of the paper and are displaced by the birefringentsecond polarization splitter 212. These are the even channels 217 havingpolarization vectors as shown. The even channels of beam 209 havingpolarization vectors 211 traverse second polarization splitter 212undisplaced and emerge as beam 218 having vector components as shown.

[0040] A half wave plate (or compensator) 219 rotates the polarizationstates beams 217 and 218 so as to minimize polarization mode dispersion.In the exemplary embodiment of FIG. 2, the half-wave plate 219 isillustratively an achromatic quartz half-wave plate. Orthogonallypolarized beams 220 and 221, containing the even channels then traversebirefringent element 222, with beam 221 being displaced and combinedwith undisplaced beam 220 to form the even channel output signal 223.(It is noted that herein, the second polarization splitter 212, thereflective element 215, the half wave plate 219 and the birefringentelement 223 may be referenced as a combining element).

[0041] As can be appreciated from a review of FIG. 2, the input signal201, having a first channel spacing, undergoes a transformation by theinterleaver 200 into odd and even output signals 216 and 223,respectively, each of which have channels with a second channel spacingthat is twice that of the channel spacing of input signal 201. Oddoutput signal 216 could be input to a concatenatedinterleaver/deinterleaver (substantially the same as interleaver 200),and the channel spacing could be increased by another factor of two.Likewise, this could be accomplished using even output signal 223 as theinput of a concatenated interleaver/deinterleaver. Of course thisconcatenation can continue, with each successive interleaver furtherincreasing the channel spacing by a factor of two. It is further notedthat the odd and even output signals 216 and 223 could also beselectively coupled to other devices such as demultiplexers and opticaladd/drops.

[0042] In the illustrative embodiment shown in FIG. 2, thede-interleaving which illustratively doubles the first channel spacingof the input signal exploits the anisotropic optical properties ofelement 205. Moreover, the element 205 is illustratively a birefringentcrystal having ordinary and extraordinary axes. In the exemplaryembodiment of FIG. 2, the polarization vectors of beams 203 and 204 arein mutually orthogonal polarization states. The c-axis of birefringentelement 205 is oriented at 45° relative to each of these polarizationstates. As such, it can be shown that for beam 203 having thepolarization state described above, the transmittance (at the output ofPT element 205) is given by: $\begin{matrix}{T = {\left( \frac{1}{2} \right)\left\lbrack {1 + {\cos \left( {\varphi_{0} + {2{\pi\upsilon\tau}}} \right)}} \right\rbrack}} & {{eqn}.\quad (1)}\end{matrix}$

[0043] Where υ is the relative optical frequency of a particularchannel, φ₀ is a phase constant, and τ is the temporal delay between theextraordinary and ordinary polarization vectors, given by:$\begin{matrix}{\tau = {{\left( \frac{L}{c} \right)\left( {n_{e} - n_{o}} \right)} = {\left( \frac{L}{c} \right)\left( {\Delta \quad n_{g}} \right)}}} & {{eqn}.\quad (2)}\end{matrix}$

[0044] where L is the length of the crystal (in this case PT element205), c is the speed of light in vacuum, and Δn_(g) is the group indexof refraction difference between the ordinary and extraordinary indicesof refraction for the center wavelength of the particular channel. Ofcourse a similar analysis would hold for beam 204, and is forgone atthis point in the interest of brevity and clarity of discussion.

[0045] The resultant temporal delay, τ, between the extraordinary andordinary beams is exploited in the present invention. To this end,(again the discussion will focus only one beam in the interest ofsimplicity), the odd channels 206 and the even channels 208, have arelative temporal delay τ as a result of traversing the crystal 205.This delay is manifest in the optical transmission coefficient forelliptically polarized beam 208. The frequency period or spacing of thesinusoidal transmission coefficient given by (1) above is$\left( \frac{1}{\tau} \right).$

[0046] Accordingly, odd channels 206 and even channels 208 have achannel spacing of $\left( \frac{1}{\tau} \right).$

[0047] It follows therefore, that after recombination of the of oddchannels (which are in orthogonal polarization (temporally retarded)states due to the birefringent properties of crystal 205) the channelspacing of the output signal at odd channel output port 216 is also$\left( \frac{1}{\tau} \right),$

[0048] twice that of the channel spacing of the input channel 201, whichis $\left( \frac{1}{2\tau} \right)$

[0049] Thus, in the illustrative embodiment of the present invention,the input optical signal 201 has a channel frequency spacing of$\left( \frac{1}{2\tau} \right),$

[0050] while the odd and even output signals 216 and 223, respectivelyeach have a channel frequency spacing of$\left( \frac{1}{\tau} \right).$

[0051] The increase in channel spacing at the output will affordbenefits in optical communication systems. A similar analysis followsfor the output signal at even channel output port 223.

[0052] In the illustrative embodiment of FIG. 2, the channel spacing(frequency spacing) that results from the birefringent characteristicsof crystal 205 can be chosen by picking the length of the crystal, L,for a particular birefringent crystal 205. Moreover, the frequencyspacing or free spectral range FSR is given by$\left( \frac{1}{\tau} \right).$

[0053] Thus, the FSR can be set by the selection of the crystal length,L, for a particular crystal. Finally, the center wavelength or frequencyof the channels can be set by selection of φ₀ in (1) above. This can aidin fine-tuning the embodiment of FIG. 2 to a standard channel grid, suchas an ITU grid mentioned above.

[0054] As referenced previously, in many instances, adverseenvironmental effects can have a deleterious impact on the performanceof optical devices. In accordance with another exemplary embodiment ofthe present invention a method and apparatus forinterleaving/de-interleaving optical signals includes a passive thermalcompensator. The passive thermal compensator enables the apparatus tooperate over a predetermined temperature range with substantially notemperature induced frequency (and, thereby, wavelength) drift of theoutput signal.

[0055] Turning initially to FIG. 3, an optical device 300 in accordancewith an illustrative embodiment of the present invention is shown. Aninput signal 305 is incident on an interleaver/deinterleaver 301(hereinafter referred to as interleaver 301). The input signal 305 is aWDM optical signal illustratively having channels 1, 2, . . . n, withrespective channel center wavelengths of λ₁, λ₂, . . . λ_(n). Outputsignals 303,304 are deinterleaved, and have a channel spacing that is anintegral multiple of the channel spacing of the input signal 305.

[0056] In the exemplary embodiment of FIG. 3, the interleaver 301includes at least one birefringent element, which is used to separatethe polarization states of the input signal 305 into orthogonalpolarization components. As is described in greater detail herein,ambient temperature affects both the optical path length and thebirefringence of birefringent elements. Accordingly, it is beneficial tocompensate for changes in the ambient temperature, which can adverselyimpact the optical signal, and to do so in a passive manner.

[0057] A passive thermal compensator 302 according to an illustrativeembodiment of the present invention compensates for thermal effects(e.g., ambient thermal effects) so that the output signals 303,304 ofthe optical device 300 experience substantially no temperature-inducedfrequency/wavelength drift compared to the input optical signal 305.According to an illustrative embodiment of the present invention, thepassive thermal compensator 302 includes at least one birefringentelement, which compensates for temperature-induced frequency orwavelength shift of the input signal 305 that is caused by thermalinfluences on the interleaver/deinterleaver 301.

[0058] Accordingly, the output signals 303 and 304 (deinterleavedsignals in this illustrative embodiment) are substantially unaffected byvariations in the ambient temperature over a predetermined range. Assuch, fluctuations in the optical network ambient temperature, which mayresult from internal and external components of the optical network, arecompensated for by virtue of the passive thermal compensator 302 withoutthe necessity for ambient temperature controls, such as coolingelements. Moreover, as will become clearer as the present descriptionproceeds, the passive thermal compensator 302 in accordance with thepresent exemplary embodiment compensates/corrects for thermal effectsrather than attempts to prevent them as an active ambient temperaturecontroller would.

[0059]FIG. 4 shows another illustrative embodiment of the invention ofthe present disclosure. An interleaver/deinterleaver 400 (hereinafterreferred to as interleaver 400) is illustratively based on polarizationinterferometry using birefringent materials and includes a passivethermal compensator. It is noted that many of the details ofde-interleaving of the input optical signal 401 are substantiallyidentical to those described in connection with the exemplary embodimentof FIG. 2. As such, many of these details may not be repeated in theinterest of brevity.

[0060] An input signal 401 is incident perpendicularly to the end faceof the polarization splitter 402, which illustratively includes abirefringent material such as rutile, calcite or yttrium vanadate(YVO₄). The polarization splitter 402 fosters polarization diversity,and ultimately enables the interleaver 400 to function independently ofthe polarization of the input signal 401. The polarization splitter 402splits the input signal 401 into two beams 403. These two beams 403 haveorthogonal polarization states, which are often referred to as s and ppolarization states having a relative phase that is determined by thethickness and material properties (birefringence) of the polarizationsplitter 402. The input optical signal 401 is multiplexed havingchannels 1, 2, . . . n, and the channels have respective channel centerwavelengths λ₁, λ₂, . . . λ_(n). The polarization splitter 402 merelysplits the electric field vector of input signal 401 into its orthogonalcomponents, the polarization states of beams 403. It follows, thereforethat each of the beams 403 contain all the channels.

[0061] Next, beams 403 are incident upon polarization transforming (PT)element 404. In the illustrative embodiment, PT element 404 is abirefringent crystal, such as calcite, rutile, or (YVO₄). It is notedthat PT element 404 may also be a known phase retarder. The physicalproperties that are desirable in PT element 404 are its opticalanisotropies for effecting the polarization transformation of beams 403.Ultimately, the optical properties of element 404 are useful inde-interleaving the input signal 401 into output optical signals havinga channel spacing that is twice that of the channel spacing of the inputsignal 401.

[0062] It is noted again that other anisotropic materials such as thosereferenced above may be used for PT element 404. Moreover, in theillustrative embodiment shown in FIG. 4, element 404 is a birefringentcrystal having its principle section and c-axis oriented diagonally (ata 45° angle) relative to end phases of the crystal. As such, the c-axisis at 45° angle relative to the plane of polarization of the beams 403.

[0063] In the illustrative embodiment shown in FIG. 4, the channelseparation that ultimately enables the de-interleaving of the inputoptical signal, exploits the anisotropic optical properties of PTelement 404. Moreover, PT element 404 is illustratively a birefringentcrystal having ordinary and extraordinary axes. In the exemplaryembodiment of FIG. 4, the polarization vectors of beams 403 are inmutually orthogonal polarization states. The c-axis of birefringentelement 404 is oriented at 45° relative to each of these polarizationstates. For beams 403 having the polarization state described above, thetransmittance of beam 407 (odd channels) is given by equation (1) above.Again, τ is the temporal delay between the ordinary and extraordinarypolarization vectors and is given by eqn.(2). Moreover, 1-T istransmitted by beam 410 (even channels)after its combination by elements406, 408 and 409.

[0064] The temporal delay, τ, between the extraordinary and ordinarybeams is exploited to effect the desired channel spacing in thede-interleaving of the input signal 401. However, as mentioned brieflyabove, the temperature of birefringent elements (such as element 404)may affect both the length and indices of refraction of the birefringentelement. Ultimately, because the channel spacing is related to thetemporal delay τ, the channel spacing may also be adversely impacted bythe affect of temperature on the optical path length and birefringence.

[0065] Quantitatively, the temperature induced frequency shift ofelement 204 is given by: $\begin{matrix}{{\frac{\partial f}{\Delta \quad T} = {\frac{c}{{\lambda\Delta}\quad n_{g}}\left( {\frac{{\Delta}\quad n}{T} + {\Delta \quad n\quad \alpha}} \right)}},{where},} & {{eqn}.\quad (3)} \\{{\frac{{\Delta}\quad n}{T} = {\frac{n_{e}}{T} - \frac{n_{o}}{T}}},} & {{eqn}.\quad (4)}\end{matrix}$

[0066] α is the thermal expansion coefficient of element 404, λ is thewavelength and ƒ is the relative optical frequency.

[0067] Illustratively, using known material properties of YVO₄, thetemperature induced wavelength drift is on the order ofapproximately-4.2 GHz/° C. This is, unfortunately, on the order 5 timesthe temperature drift of an uncompensated fiber Bragg grating, which maybe used in the de-interleaving of a WDM/DWDM signal. In order forWDM/DWDM devices based on anisotropic optical elements such asbirefringent crystals to be practical in deployed optical systems, it isimportant to reduce the thermal drift either by active temperaturecontrol (such as climate control elements) or by passive compensation.As discussed above, active temperature control techniques have certaindrawbacks making the passive compensation technique of the presentinvention an attractive alternative to active control of the ambienttemperature.

[0068] As stated above, the thermal effects on the birefringent elementssuch as PT element 404 are generally manifest in a change in the length(L) of the PT element 404 as well as a change in indices of refractionalong the ordinary and extraordinary axes. These thermal effects must becompensated for, as a temperature induced change in these parametersultimately affects the temporal delay τ, the free spectral range (FSR),and the channel spacing. According to illustrative embodiments of thepresent invention, the thermal compensation is effected passively,incorporating a compensating anisotropic optical element such as passivethermal compensating (PTC) element 405 to effect temperaturecompensation. Moreover, PTC element 405 is adjacent PT element 404 andelements 404 and 405 are illustratively birefringent materials, whichare adhered to one another by suitable adhesive. As such, PT element 404and PTC element 405 behave substantially as one optical element.

[0069] As stated, an object of the present invention is to minimize thetemperature induced wavelength drift. Accordingly, one objective of thepresent invention is that the temperature induced frequency shift ofelement 404 is nullified. Quantitatively, this means that eqn. (3) iszero: $\begin{matrix}{\frac{\partial f}{\Delta \quad T} = O} & {{eqn}.\quad (5)}\end{matrix}$

[0070] In the exemplary embodiment of FIG. 4, where a compensatingelement such as PTC element 405 is used to achieve passive thermalcompensation, it can be shown that thermal compensation requires:$\begin{matrix}{{{L_{1}\frac{{\Delta}\quad n_{1}}{T}} + {L_{2}\quad \frac{{\Delta}\quad n_{2}}{T}} + {\Delta \quad n_{1}L_{1}\alpha_{1}} + {\Delta \quad n_{2}L_{2}\alpha_{2}}} = O} & {{eqn}.\quad (6)}\end{matrix}$

[0071] where L₁ is the length of PT element 404, L₂ is the length ofelement 405, Δn₁ is the birefringence of element 404, Δn₂ is thebirefringence of element 405, α₁ is the expansion coefficient of element404 and α₂ is the expansion coefficient for element 405. This isreferred to as the condition of athermalization.

[0072] Other objectives of the passive thermal compensation of thepresent invention are to maintain the channel spacing, the temporaldelay τ, and free spectral range (FSR across a particular temperaturerange. The temporal delay, τ, of birefringent elements 404 and 405 areadditive. As such:

τ=τ₁+τ₂   eqn.(7) $\begin{matrix}{\tau = {{\frac{L_{1}}{c}\left( {\Delta \quad n_{1}} \right)} + {\frac{L_{2}}{c}\left( {\Delta \quad n_{2}} \right)}}} & {{eqn}.\quad (8)}\end{matrix}$

[0073] where τ₁ and τ₂ are the temporal delays resulting frombirefringent elements 204 and 405, respectively.

[0074] Because the temperature dependence of the indices of refractionof the birefringent elements, as well as the expansion coefficients α₁,α₁ of elements 404 and 405, respectively, are known, in order to satisfythe above relations of eqn. (6) and (8), it is necessary to determinethe suitable lengths L₁ and L₂ for the particular materials chosen forbirefringent elements 404 and 405. Practically, this requiresdetermining the ratio of $\frac{L_{1}}{L_{2}}$

[0075] from eqn. (6), the condition for athermalization. Accordingly,once this ratio is determined, the interleaver 400 is passivelyathermalized for a variety of free spectral ranges (FSR), and channelspacing. Stated differently, when the ratio of the lengths of elements404 and 405 is determined from eqn. (6), the interleaver 400 accordingto an illustrative embodiment of the invention of the present disclosureis athermalized for free spectral ranges and channel spacings ofinterest. It follows of course that if it is desired to have anathermalized interleaver with a particular free spectral range/channelspacing, it is necessary to determine the appropriate ratio of thelengths of elements 405 and 405. Illustratively, the interleaver 400 mayinterleave/deinterleave optical signals having free spectral ranges (andchannel spacings) of 400 GHz, 200 GHz, 100 GHz, 50 GHz and 25 GHz, and12.5 GHz in an athermalized manner.

[0076] It is noted that according to the presently described exemplaryembodiment, the fast axes of PT element 404 and PTC element 405 shouldbe either perpendicular or parallel to one another. If the fast axis ofPT element 404 were oriented at an angle between 0° and 90°, andmultiples thereof, the output would not be the same as the idealsituation where there is no need to compensate for thermal effects withelement 405. To this end, if the fast axes of the PT element 404 and PTCelement 405 are not oriented parallel or perpendicular to one another,the phase delay between the extraordinary and ordinary rays will resultin harmonics and the output will not be the desired output, which isillustratively the deinterleaved output signal described in the parentapplication.

[0077] According to an illustrative embodiment of the present invention,the crystal that may be used for PTC element 405 has a fast axis, whichis parallel to the fast axis of PT element 404, which is alsoillustratively a birefringent crystal. Moreover, the crystal should bechosen so that$\frac{{\Delta}\quad n_{2}}{T}\quad {and}\quad \frac{{\Delta}\quad n_{1}}{T}$

[0078] have opposite signs. Since $\frac{{\Delta}\quad n_{1}}{T}$

[0079] is usually the dominating term, it is possible to cancel thethermal affect while the birefringence of the second crystal is merelyadditive to that of the first crystal. In this particular embodiment,the length of the PT element 404 (also referred to as the dominatingelement) is shorter than the birefringent element of the uncompensatedinterleaver, as described in the parent application. Again, this isbecause the birefringence is additive in this particular embodiment.Accordingly, the above described embodiment enables the passivecompensation for thermal effects by determination of the lengths L₁ andL₂, which satisfy the relation of eqn. (6).

[0080] Alternatively, the fast axes of PT element 404 and PTC element405 may be orthogonal. Accordingly, the fast axis of the first element404 will be parallel to the slow axis of element 405. In this case, andparticularly when yttrium vanadate (YVO₄) is used for birefringentelements 404 and 405, $\frac{{\Delta}\quad n_{2}}{T}$

[0081] has the same sign as $\frac{{\Delta}\quad n_{1}}{T},$

[0082] but$\frac{\Delta \quad n_{1}}{\Delta \quad n_{2}}{\frac{\frac{{\Delta}\quad n_{1}}{T}}{\frac{{\Delta}\quad n_{2}}{T}}.}$

[0083] As such, not only does the second birefringent element 405compensate for the thermal effect, but it also reduces the amount ofbirefringence provided by the first element 404. This partialcancellation of the birefringence of the first element 404 by the secondelement 405 necessitates, of course, that the first element 404 has agreater length (L₁) than in the case in which there is no compensationfor thermal effects. By solving the condition for athermalization as setforth in eqn. (6), the ratio of the lengths$\left( \frac{L_{1}}{L_{2}} \right)$

[0084] PT element 404 to the PTC element 405 may be determined.Moreover, by solving eqn. (6) and (8,) the lengths L₁ and L₂ forelements 404 and 405, respectively, for particular materials, temporaldelay, FSR and channel spacing may be determined in absolute value.Suitable materials such as lithium niobate (LiNbO₃) may be used forelement 405 in this illustrative embodiment, with YVO₄ illustrativelybeing used as the material for PT element 404. Of course, the materialsused for elements 402, 404, 405 and 406 could be other anisotropicoptical elements such as those described previously.

[0085] As can be readily appreciated, the embodiment shown in FIG. 4provides passive thermal compensation to a WDM optical link. Dependingupon applications, residual temperature drift in WDM optical link basedon polarization interferometry may be required to be less thanapproximately ±1 GHz over an illustrative temperature range ofapproximately −5° C. to approximately +70° C. This is an improvement onthe order of approximately a factor of 170 compared to an uncompensatedpolarization interferometry based WDM, and is comparable to theperformance of a fiber Bragg grating (FBG) based interleaver.

[0086] In some circumstances, it may be desirable to compensate forsecond order temperature effects. According to an exemplary embodiment,this may be carried out using an additional PTC element. Such an elementis shown in FIG. 5 at 501. To this end, element 500 includes PT element404, and PTC element 405 of FIG. 4 as well as another PTC element 501.The basic teachings of FIG. 4 apply to FIG. 5, and the other elements ofthe interleaver 400 of FIG. 4 have been forgone in FIG. 5 in theinterest of clarity of discussion. (Of course, element 500 would belocated between element 402 and element 406 in interleaver 400).

[0087] For precise passive thermal compensation a detailed knowledge ofthe thermal properties of elements 404, 405 and 501 is useful.Fortunately, the interleaver 400 shown generically in FIG. 4 is a verygood temperature sensor. By measuring the temperature dependence of thefrequency (wavelength) on shift in a polarization independentinterleaver 400, which contains at least one birefringence element,higher order temperature coefficients can be derived. As such, thesecond order elements can be used in combination with the first orderelements and the basic equation for the temporal delay, τ in order todetermine the suitable ratios for the lengths (L₁, L₂, L₃) of elements404, 405 and 501, respectively, for various materials used in thesecapacities.

[0088] Quantitatively it can be shown that: $\begin{matrix}{\tau = {{\frac{L_{1}}{C}\left( {\Delta \quad n_{1}} \right)} + {\frac{L_{2}}{C}\left( {\Delta \quad n_{2}} \right)} + {\frac{L_{3}}{C}\left( {\Delta \quad n_{3}} \right)}}} & {{eqn}.\quad (9)} \\{{{L_{1}\frac{\left( {\Delta \quad n_{1}\alpha_{1}} \right)}{T}} + {L_{2}\frac{\left( {\Delta \quad n_{2}\alpha_{2}} \right)}{T}} + {L_{3}\frac{\left( {\Delta \quad n_{3}\alpha_{3}} \right)}{T}}} = O} & {{eqn}.\quad (10)} \\{{{{L_{1}\frac{^{2}}{T^{2}}\left( {\Delta \quad n_{1}\alpha_{1}} \right)} + {L_{2}\frac{^{2}}{T^{2}}\left( {\Delta \quad n_{2}\alpha_{2}} \right)} + {L_{3}\frac{^{2}}{T^{2}}\left( {\Delta \quad n_{3}\alpha_{3}} \right)}} = O},} & {{eqn}.\quad (11)}\end{matrix}$

[0089] where L₃, Δn₃, α₃ are the length, birefringence, and expansioncoefficient, respectively, of the third element 501. Finally, theorientation of the fast axes of elements 404, 405 and 501 describedabove are illustrative. As would readily be appreciated by one ofordinary skill in the art having had the benefit of the presentdisclosure, the orientation of the fast axes of elements 404, 405 and501 can be a variety of permutations of parallel and perpendicularorientations. While not particularly spelled out in the presentinvention, these are, of course, within the scope of the presentinvention.

[0090] The invention having been described in detail in connectionthrough a discussion of exemplary embodiments, it is clear that variousmodification of the invention will be apparent to one of ordinary skillin the art having had the benefit of the present disclosure. Suchvariations and modification are included within the scope of theappended claims.

1. An optical device, comprising: An interleaver/deinterleaver includinga passive thermal compensator, wherein an optical signal which traversesthe optical device undergoes substantially no temperature-inducedfrequency drift over a desired temperature range.
 2. An optical deviceas recited in claim 1, wherein said thermal compensator further includesat least one birefringent element.
 3. An optical device as recited inclaim 1, wherein said interleaver/deinterleaver further includes atleast one birefringent element.
 4. An optical device as recited in claim3,wherein said at least one birefringent element of said interleaver hasa fast axis; said at east one birefringent element of said thermalcompensator has a fast axis, and said fast axes are orthogonal.
 5. Anoptical device as recited in claim 3, wherein saidinterleaver/deinterleaver further comprises at least one birefringentelement that has a fast axis, and said at least one birefringent elementof said thermal compensator has a fast axis, and said fast axes areparallel.
 6. An optical device as recited in claim 1, wherein saidthermal compensator includes at least two birefringent elements.
 7. Anoptical device as recited in claim 3, wherein said at least onebirefringent element of said thermal compensator is attached to saidbirefringent element of said interleaver.
 8. An optical device asrecited in claim 6, wherein said at least two birefringent elements ofsaid thermal compensator are attached to one another and one of said atleast two birefringent elements is attached to said birefringent elementof said interleaver.
 9. An optical device as recited in claim 1, whereinsaid temperature induced frequency drift is in the range ofapproximately −2.5 GHz to approximately +2.5 GHz.
 10. An optical deviceas recited in claim 1, wherein said desired temperature range is about80° C.
 11. An optical device as recited in claim 3, wherein said atleast one birefringent element of said interleaver/deinterleaver ischosen from the group consisting essentially of rutile, calcite, lithiumniobate and yttrium vanadate.
 12. An optical device as recited in claim3, wherein said birefringent element has a length L₁ and said at leastone birefringent element has a length L₂.
 13. An optical device asrecited in claim 1, wherein said desired temperature range isapproximately −10° C. to approximately +70° C.
 14. An optical device asrecited in claim 1, wherein the optical device satisfies a condition ofathermalization given by:${{L_{1}\frac{{\Delta}\quad n_{1}}{T}} + {L_{2}\frac{{\Delta}\quad n_{2}}{T}} + {\Delta \quad n_{1}L_{1}\alpha_{1}} + {\Delta \quad n_{2}L_{2}\alpha_{2}}} = O$

where L₁ is a length of said at least one birefringent element of saidinterleaver/deinterleaver, L₂ is a length of said at least onebirefringent element of said thermal compensator, Δn₁ is thebirefringence of said at least one birefringent element of saidinterleaver/deinterleaver, Δn₂ is the birefringence of said at least onebirefringent element of said thermal compensator, α₁ is an expansioncoefficient of said at least one birefringent element of saidinterleaver, and α₂ is the expansion coefficient of said at least onebirefringent element of said thermal compensator
 15. An optical deviceas recited in claim 1, wherein said input signal includes mxn channels(m,n=integer), and said at least one output signal includes n/m outputchannels.
 16. An optical signal as recited in claim 14, wherein saidoutput channels have a spacing in the range of approximately 12.5 GHz toapproximately 400 GHz.
 17. An optical device as recited in claim 15,wherein said temperature induced frequency drift is in the range ofapproximately −2.5 GHz to approximately +2.5 GHz.
 18. An optical deviceas recited in claim 15, wherein m=2.
 19. A method ofinterleaving/deinterleaving an optical signal, the method comprising:providing an interleaver/deinterleaver, which includes a passive thermalcompensator, wherein an optical signal which traverses the opticaldevice undergoes substantially no temperature-induced frequency driftover a desired temperature range.
 20. A method as recited in claim 19,wherein said thermal compensator further includes at least onebirefringent element.
 21. A method as recited in claim 20, wherein saidinterleaver/deinterleaver further includes at least one birefringentelement.
 22. A method as recited in claim 21 ,wherein said at least onebirefringent element of said interleaver has a fast axis; said at leastone birefringent element of said thermal compensator has a fast axis,and said fast axes are orthogonal.
 23. A method as recited in claim 19,wherein said interleaver/deinterleaver further comprises at least onebirefringent element that has a fast axis, and said at least onebirefringent element of said thermal compensator has a fast axis, andsaid fast axes are parallel.
 24. A method as recited in claim 19,wherein said passive thermal compensator further comprises at least twobirefringent elements.
 25. A method as recited in claim 19, wherein saiddesired temperature range is about 80° C.
 26. An apparatus for opticalinterleaving/de-interleaving, comprising: an input port which receivesan optical signal, said optical signal being polarized light and havingmultiple channels; a first element which decomposes said optical signalinto a first beam and a second beam, said first beam being in a firstpolarization state and said second beam being in a second polarizationstate, said first and said second polarization states being orthogonalto one another and including each of said multiple channels; a secondelement which transforms said first beam into a first ellipticallypolarized state having odd channels in a third polarization state andeven channels in a fourth polarization state, and said second elementtransforming said second beam into a second elliptically polarized statehaving even channels in said third polarization state and odd channelsin said fourth polarization state; and a third element which combinessaid odd channels into a first output port and said even channels into asecond output port.
 27. An apparatus as recited in claim 26, whereinsaid multiple channels have a channel spacing of (½τ) and said oddchannels have a channel spacing of (1/τ) and said even channels have achannel spacing of (1/τ).
 28. An apparatus as recited in claim 26,wherein the apparatus is concatenated with a second apparatus and athird apparatus, each of which further comprise aninterleaver/deinterleaver and said multiple channels are selectivelyoutput from said second apparatus and said third apparatus having achannel spacing (2/τ).
 29. An apparatus as recited in claim 26, whereinsaid second element is a birefringent material.
 30. An apparatus asrecited in claim 26, wherein said first element is a polarizationsplitter.
 31. An apparatus as recited in claim 26, wherein said opticalsignal is elliptically polarized.
 32. An apparatus as recited in claim26, wherein said third element includes a polarization splitter.
 33. Anapparatus as recited in claim 26, wherein said third element furtherincludes a half-wave plate and a birefringent element.
 34. An apparatusas recited in claim 27, wherein (½τ) is 400 GHz.
 35. An apparatus asrecited in claim 27, wherein (½τ) is 200 GHz.
 36. An apparatus asrecited in claim 27, wherein (½τ) is 100 GHz.
 37. An apparatus asrecited in claim 27, wherein (½τ) is 50 GHz.
 38. An apparatus as recitedin claim 27, wherein (½τ) is 25 GHz.
 39. An apparatus as recited inclaim 21, wherein (½τ) is 12.5 GHz.
 40. An apparatus forinterleaving/de-interleaving optical signals, comprising: an input portwhich receives an elliptically polarized optical signal; a polarizationsplitter which splits said elliptically polarized optical signal stateand a second beam being in a second polarization state, said first andsecond polarization states being mutually orthogonal; a birefringentelement which transforms said first beam into a first ellipticallypolarized state having odd channels in a third polarization state andeven channels in a fourth polarization state, and said birefringentelement transforming said second beam into a second ellipticallypolarized state having even channels in said third polarization stateand odd channels in said fourth polarization state; and a combiningelement which combines said odd channels into a first output ort andsaid even channels at a second output port.
 41. An apparatus as recitedin claim 40, wherein said optical signal includes multiple channelshaving a first channel spacing, and said odd channels and said evenchannels each have a second channel spacing, wherein said second channelspacing is twice said first channel spacing.
 42. An apparatus as recitedin claim 40, wherein said first element is a polarization splitter. 43.An apparatus as recited in claim 40, wherein said birefringent elementis chosen from the group consisting essentially of electro-opticdevices, birefringent crystals and birefringent optical fibers.
 44. Anapparatus as recited in claim 41, wherein said first channel spacing is400 GHz.
 45. An apparatus as recited in claim 41, wherein said firstchannel spacing is 200 GHz.
 46. An apparatus as recited in claim 41,wherein said first channel spacing is 100 GHz.
 47. An apparatus asrecited in claim 41, wherein first channel spacing is 50 GHz.
 48. Anapparatus as recited in claim 41, wherein first channel spacing is 25GHZ.
 49. An apparatus as recited in claim 41, wherein first channelspacing is 12.5 GHz.
 50. An apparatus as recited in claim 41, whereinsaid combining element includes a half wave plate and a polarizationsplitter.
 51. An apparatus as recited in claim 40, wherein saidbirefringent element has a length, L, given by: L=(τ/c)/(Δn _(g))
 52. Anapparatus as recited in claim 40, wherein said birefringent element hasa c-axis (optic axis) oriented at 45° to said first and said secondpolarization states.
 53. A method of interleaving/de-interleaving anoptical signal, the method comprising: providing an optical signal to aninput port, said optical signal being polarized light and includingmultiple channels; decomposing said optical signals into a first beamand a second beam said first beam being in a first polarization stateand said second beam being in a second polarization state, said firstand said second polarization states being orthogonal to one another andincluding each of said multiple channels; transforming said first beaminto a first elliptically polarized state having odd channels in a thirdpolarization state and even channels in a fourth polarization state andtransforming said second beam into a second elliptically polarized statehaving even channels in said third polarization state and odd channelsin said fourth polarization state; and combining said odd channels intoa first output port and said even channels into a second output port.54. A method as recited in claim 53, wherein said multiple channels havea channels spacing of (½τ) and said odd channels have a spacing of (1/τ)and said even channels have a spacing of (1/τ).
 55. A method as recitedin claim 54, wherein (½τ) is 400 GHz.
 56. A method as recited in claim54, wherein (½τ) is 200 GHz.
 57. A method as recited in claim 54,wherein (½τ) is 100 GHz.
 58. A method as recited in claim 54, wherein(½τ) is 50 GHz.
 59. A method as recited in claim 54, wherein (½τ) is 25GHz.
 60. A method as recited in claim 54, wherein (½τ) is 12.5 GHz. 61.A method as recited in claim 53, wherein the method further comprises:providing said odd channels and said even channels to a concatenatedsecond interleaver/deinterleaver and a concatenated thirdinterleaver/deinterleaver, respectively, and said even and said oddchannels are selectively output from said second and said thirdmultiplexer/demultiplexers having a channel spacing (2/τ).